הם לא מכירים אותי, אבל את מכירה אותם.

They are related, but they are not doing the same job in exactly the same way.

• את in אבל את מכירה אותם means you (addressing one female).
• אותי means me.
• אותם means them.

The forms אותי, אותך, אותו, אותה, אותנו, אתכם, אתכן, אותם, אותן are object forms built from the direct-object marker את plus pronoun endings.

So:

• מכירים אותי = know me
• מכירה אותם = know them

Important: the standalone word את can mean either:

1. you (feminine singular), or
2. the direct-object marker et

In this sentence, את is the subject pronoun you.

Because the verb changes to match the subject in number and sometimes gender.

Here the verb is from להכיר (to know / to be acquainted with / to recognize, depending on context).

Present tense forms:

• מכיר = masculine singular
• מכירה = feminine singular
• מכירים = masculine plural (or mixed group)
• מכירות = feminine plural

So:

• הם לא מכירים אותי = They do not know me
  because הם = they (masculine plural / mixed group)

• את מכירה אותם = You know them
  because את = you addressed to one female

You would use the masculine singular form:

הם לא מכירים אותי, אבל אתה מכיר אותם.

Changes:

• את → אתה = you (masculine singular)
• מכירה → מכיר to match a masculine singular subject

That is the normal way to make a sentence negative in Hebrew.

• הם מכירים אותי = They know me
• הם לא מכירים אותי = They do not know me

So לא simply comes before the verb to negate it.

להכיר can have a few related meanings, and context decides which one is intended.

Common uses:

• to know / be acquainted with a person
• to recognize
• to get to know / become acquainted with in some structures

In this sentence, מכירים אותי and מכירה אותם most naturally mean:

• they don't know me
• but you know them

That is, to know people / be acquainted with them.

Hebrew often uses להכיר for knowing people, while לדעת is more often used for knowing facts or information.

For example:

• אני מכיר אותו = I know him
• אני יודע את התשובה = I know the answer

The sentence is built as two clauses joined by אבל (but):

• הם לא מכירים אותי
• אבל את מכירה אותם

This is very natural Hebrew word order.

Literally:

• They not know me, but you know them.

Hebrew often uses a straightforward subject–negation–verb–object order here.

Yes, Hebrew often can leave subject pronouns out, because the verb already shows person, number, and sometimes gender.

For example, Hebrew could say:

• לא מכירים אותי, אבל מכירה אותם

But in your sentence, the pronouns are included because they make the contrast clearer:

• הם don't know me,
• but את know them.

So the pronouns help emphasize the comparison between they and you.

Because Hebrew, like English, uses different forms for subjects and objects.

• הם = they (subject form)
• אותם = them (object form)

Compare:

• הם מכירים אותי = They know me
• את מכירה אותם = You know them

Just like English uses they vs them, Hebrew uses הם vs אותם.

Grammatically, הם is the masculine plural form, but in real usage it can also refer to a mixed group of males and females.

If the group were entirely female, standard Hebrew would use:

• הן לא מכירות אותי, אבל את מכירה אותן.

So:

• הם / אותם / מכירים = masculine plural or mixed group
• הן / אותן / מכירות = feminine plural

Yes, at first it can look confusing to learners, but the grammar makes it clear.

In אבל את מכירה אותם:

• את is a subject pronoun meaning you (female singular)
• מכירה is a feminine singular verb, which confirms that את must be the subject

If את were the direct-object marker, it would normally be followed by a definite noun, not by a verb form that agrees with it as a subject.

So once you notice the agreement in מכירה, the sentence becomes clear.

A very close literal breakdown is:

• הם = they
• לא = not
• מכירים = know
• אותי = me
• אבל = but
• את = you (feminine singular)
• מכירה = know
• אותם = them

So the sentence is:

They do not know me, but you know them.

That is also a very natural translation, not just a literal one.